Polycrystalline silicon as carrier selective contact for silicon solar cells · 2018. 8. 12. · Polycrystalline silicon as carrier selective contact for silicon solar cells SPREE

Polycrystalline silicon as carrier selective

contact for silicon solar cells

SPREE Seminar Talk

Udo Römer

21.07.2016

Outline

Process optimisation

poly-Si contacts

Theory / understanding

poly-Si contacts

Local overcompensation

via ion implantation

Combination of these

technologies

1

Motivation

• Removing front side metal shadowing

~2.5 mA/cm² gain in Jsc

• Voltage can be enhanced by reducing

recombination:

• SP-PERC cell has J0 of ~300 fA/cm²

• Poly-Si / c-Si-junction:

15 fA/cm² for p-type poly-Si1 and

20 fA/cm² for n-type poly-Si1

~70 mV gain in Voc

0

ln 1gen

OC

JkTV

q J

PERC solar cell

Poly-Si rear contact

solar cell

[1] J. Y. Gan and R. M. Swanson, IEEE Trans. Electron Devices, vol. ED-37, pp.245-250 (1990) 2

Passivation:

• Low Dit at wafer surface due to high

quality oxide

• Field effect passivation due to

highly doped poly-Si layer

Transport:

• Tunnel transport through the oxide1

or

• holes in the oxide2

Poly-Si contact: working principle

Band diagram of a n-typ poly-Si contact

(sketch)

[1] Steinkemper, Feldmann, Bivour, and Hermle, IEEE Journal of Photovoltaics, vol. 5, no. 5, pp. 1348 (2015)

[2] Peibst, Römer, Hofmann, Lim, Wietler, Krügener, Harder, and Brendel, IEEE Journal of Photovoltaics, vol. 4, no. 3, pp. 841 (2014)

electron transport

few interface

defects

interface oxide

many

defects

interface oxide:

larger tunnel barrier

for holes than for

electrons

Depth [nm]

Energ

y [

eV

]

3

Poly-Si contact: transport

RCA-oxide after 10 min at 950 °C RCA-oxide after 10 min at 1100 °C

• HR-TEM investigations show braking up of the oxide after high

temperature processing

4

Wolstenholme, Jorgensen, Ashburn, and Booker, J. Appl. Phys. 61, 225 (1987)

Process flow poly-Si test structures 1-step-process / 2-step-process

Thin oxidation

Deposition of doped poly-Si layers

Annealing / cracking of the oxide

5

Thin oxidation

Deposition of undoped poly-Si layers

Annealing / cracking of the oxide

Doping

Process flow poly-Si test structures 1-step-process / 2-step-process

5

Measurement of the recombination

characteristics

• J0-determination with photoconductance measurement and Kane &

Swanson method1

Flash lamp

Coil J0 = 5 fA/cm²

2.4 nm thermal oxide

30 min at 1050 °C

J0-test structure

6

[1] Kane and Swanson, Proc.of the 18th IEEE PVSC, pp. 578–583 (1985)

If oxide is perfectly insulating: R4PP ~ Rpoly

If oxide is perfectly conducting: R4PP ~ Rabs

Determination of the contact

resistance

If oxide is perfectly insulating: R4PP ~ Rpoly rel ~ 1

If oxide is perfectly conducting: R4PP ~ Rabs rel ~ 0

Rpoly

Rbulk

Rpoly

Rpoly

Rbulk

Rpoly

Rpoly

Rbulk

Rpoly

Rabs

4PP sheet resistance measurement Inductive sheet resistance

measurement

7

Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014.

Simulation of 4PP-measurements

Input parameters:

• Sample geometry

• Resistivities of wafer, poly-Si layer and oxide

Result:

• “Measured" sheet resistance

SENTAURUS-DEVICE

3D-simulation

8


Simulation of 4PP-measurements

• Calculation of relative contact

resistance from simulated 4PP

resistance

• Plot vs. “real” (specified) contact

resistance

• Example: poly-Si layer with sheet

resistance of 280 Ω/:

rel < 0.05 corresponds to contact

resistance of < 0.5 Ωcm²

9


Investigation of different oxides

• All oxides reach J0-values between 5 and 20 fA/cm²

• High temperatures needed for low contact resistance

• Boron-doped poly-Si contacts comparable to Phosphorus-doped 10


Investigation of different oxides

• Passivation stable up to 60 min annealing at 1050 °C

• Contact resistance decreases with increasing annealing duration

• Good combination of low J0 and rel possible 11


Influence of the metallization

• ILM measurements before and after metallization show comparable lifetime

level (apart from edge effects)

• Planar solar cell demonstrators show Voc of 714 mV1

• Series resistance of 0.6 Ωcm² not limited by poly-Si contact1

Lifetime distribution measured via Infrared

Lifetime Mapping (ILM)

Metallized pieces

Solar cell demonstrator

12

[1] Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014.


poly-Si contacts


poly-Si contacts



13

Local counterdoping with in-situ

patterned ion implantation

• Counterdoping: Overcompensating one polarity

of dopants with dopants of the other polarity

• With in-situ patterned counterdoping local doping

possible without structured dielectric layers

• Counterdoping with in-situ masked ion

implantation enables elegant process flow for

back contacted solar cells

• Process results in formation of lateral pn-junction

with heavily doped p- and n-regions

- Risk for band to band tunneling

- Risk for trap-assisted tunneling

14

Full area counterdoping

• Processing:

- 1.5 x 1015 cm-2 B implantation

- 3 x 1015 cm-2 P implantation

- Annealing at 1050 °C

• SIMS measurement:

- Phosphorus profile covers boron

profile over whole depth

Counterdoping works fine!

Römer, Peibst, Ohrdes, Larionova, Harder, Brendel, Grohe, Stichtenoth, Wütherich, et al., Proc. 39th ,IEEE PVSC, pp. 1280 (2013) 15

Local counterdoping

• Simulations featuring lateral pn-junction

• Variation of the lifetime in the “implanted” area

• Measurements on test structures and comparison with simulations

Simulations of the diode

characteristics

Measurements on test structures

p-type silicon

Full area boron

implantation

Local phosphorus

implantation

Aluminum contacts

Passivation layer

Position x [µm]

Po

sit

ion

z [

µm

]


Local counterdoping

• Simulations show no detrimental influence of highly doped pn-junction:

n = 1 as long as implant damage is well annealed

• Otherwise strong recombination in space charge region with n = 2

• Measurements on test structures show n = 1 Everything is fine!

Simulations of the diode

characteristics

Measurements on test

structures


Local counterdoping

• Further investigations (incl. influence of the

lateral doping profile & characteristics in

reverse direction) published1

Everything is fine!

• Large area (156 mm x 156 mm psq.) ion

implanted IBC solar cells featuring local

counterdoping reach efficiencies of 22.1 %2

[1] Römer, Peibst, Ohrdes, Larionova, Harder, Brendel, Grohe, Stichtenoth, et al., Proc. 39th ,IEEE PVSC, pp. 1280 (2013)

[2] Bosch Solar Energy, ISFH, press release, Aug. 15th, 2013 18


poly-Si contacts


poly-Si contacts



Combination of these

technologies

19

Process flow for ion implanted

poly-Si with counterdoping

Thermal

oxidation

LPCVD poly-Si

deposition

Boron

implantation

Annealing/

oxide break-up

Boron implanted

Test structure

Phosphorus

implanted test

structure

20



Annealing/

oxide break-up

Thermal

oxidation

LPCVD poly-Si

deposition

Boron

implantation

Masked

phosphorus

implantation

Boron implanted

Test structure

Phosphorus

implanted test

structure

Test structure

full area

counterdoping

20



Annealing/

oxide break-up

Thermal

oxidation

LPCVD poly-Si

deposition

Boron

implantation

Masked

phosphorus

implantation

Boron implanted

Test structure

Phosphorus

implanted test

structure

Test structure

local

counterdoping

Test structure

full area

counterdoping

20

Ion implantation in poly-Si

• Decreasing J0 with increasing dose

• Very low values of 1.1 fA/cm² (phosphorus) and 4.4 fA/cm² (boron)

• Increase at too high doses, especially for boron doping

Römer, Peibst, Ohrdes, Lim, Krügener, Wietler, and Brendel, IEEE Journal of Photovoltaics, vol. 5, no. 2, pp. 507 (2015) 21

Ion implantation in poly-Si

• Doping concentration constant inside poly-Si layer

• Oxide acts as diffusion barrier, in particular for phosphorus

• For high doses strong diffusion of boron into wafer 22

Römer, Peibst, Ohrdes, Lim, Krügener, Wietler, and Brendel, IEEE Journal of Photovoltaics, vol. 5, no. 2, pp. 507 (2015)

Recombination characteristics of

boron-implanted test structure

• Implantation dose: 1x1015cm-2 B

• Highest Voc,impl. value reported so far for

p+ doped poly-Si junctions

Best value F-ISE1: 694 mV

• High pFFimpl.-value of 84.6 %

(ideal value for the given Voc is 85%)


[1] Feldmann, Müller, Reichel, and Hermle, Phys. Stat. Sol. RRL, pp. 1 (2014) 23


phosphorus-implanted test structure

• Implantation dose: 5 x 1015 cm-2 P

• Very high Voc,impl. of 742 mV

• Due to J0,poly of only 1.1 fA/cm² and very

high bulk lifetime, recombination

characteristics at MPP dominated by

Auger recombination

nAuger ≈ 2/3 results in very high pFFimpl.


Counterdoping in poly-Si

• J0-values comparable to values without counterdoping

• Contact resistance of some samples very high

• For others comparable to samples without counterdoping


Counterdoping in poly-Si

• For some samples diffusion of boron into the wafer

No contact between n+ poly-Si and n-type wafer

• High phosphorus doses prevent in-diffusion of boron



counterdoped test structure

• Implantation doses: 1x1015cm-2 B

5x1015cm-2 P

• Very high Voc,impl.-value

Best value F-ISE: 682 mV1

• Despite J0,poly-value of 0.9 fA/cm²

pFFimpl.-value of “only” 84.7%

27

[1] C. Reichel, F. Feldmann, R. Müller, A. Moldovan, M. Hermle, and S. W. Glunz, 29th EUPVSEC (2014)


masked counterdoped test structure

• Implantation doses: 1x1015cm-2 B

5x1015cm-2 P

• Curve only fitable by adding a further

recombination path with n>2

• Standard SRH-Theory: 1<n<2 in SCR

Non-standard behavior e.g. coupled

defects1

e

h

h

h

h h

e

e

e

e

h h e e

h

h h e e e

h h

e


[1] Steingrube, Breitenstein, Ramspeck, Glunz, Schenk, and Altermatt, Journal of Applied Physics, vol. 110, no. 1 (2011) 28


masked counterdoped test structure

JL

Jcell

Jpara Rpara

Rs

e

h

h

h

h h

e

e

e

e

h h e e

h

h h e e e

h h

e


Reduction of pn-junction

recombination

[1] Peibst, Römer, Patent application

[2] Rienäcker, Merkle, Römer, Kohlenberg, Krügener, Brendel, and Peibst, 6th SiliconPV Conference 2016

• Lowering of poly-Si thickness

- Possibly problems with metallisation

• Adaption of implantation parameters

- Lower dose at pn-junction

- Not very helpful (see thesis)

• Removal of lateral pn-junction

- Oxidisation1

- Wet chemical etching

η = 23.9 %2

Voc = 722 mV2

FF = 78.7 %2

e

h

h

h

h h

e

e

e

e

h h e e

h

h h e e e

h h

e

JL

Jcell

Jpara Rpara

Rs

30

Summary

Fast and non-destructing method for the

estimation of the contact resistance between

different layers

Poly-Si contacts with J0-values of 0.66 fA/cm²

and 4.4 fA/cm² for phosphorus and boron

doping developed

Full poly-Si contacted solar cell with Voc of

714 mV and low contact resistance fabricated

31

Summary

Local overcompensation via masked ion

implantation investigated

Overcompensated poly-Si contacts with

J0-values of 0.9 fA/cm² fabricated

Anomalous recombination behaivour in

locally overcompensated poly-Si contacts

investigated

32

Many thanks to

• You for your attention

• Many colleagues at ISFH and LUH for their help:

Tobias Wietler, Robby Peibst, Susanne Mau, Heike Kohlenberg, Miriam

Berger, Tobias Ohrdes, Michael Häberle, Jan Krügener, Agnes Merkle,

Bianca Lim, Yevgeniya Larionova, Sarah Spätlich, David Sylla, …

…and everyone else for the always nice atmosphere

• The Laboratory for Nano- and Quantum Engineering Hanover

• The BMWi for funding

Back-up slides

Influence of process sequence

Solar cell results

• Best process used for production of proof of principle solar cells

• High implied Voc measured; implied PFF nearly ideal (85.0 %)

impl. Voc = 716 mV

impl. PFF = 84.5 % full area wafer

Solar cell results

impl. Voc = 716 mV

impl. PFF = 84.5 % full area wafer

impl. Voc = 714 mV

impl. PFF = 72.7 %

Voc = 705 mV

PFF = 73.1 %

• Voc reduced further due to the edge effects (measured through 2 x 2 cm² mask)

Measurement without mask yields Voc of 714 mV

• Large area solar cells would not suffer from this effect

laser-cut into a

2.5 x 2.5 cm² piece

Solar cell results

• Flat surface of the solar cell and rather thick poly-Si layer (> 200 nm thick)

Low Jsc

• Edge recombination

Low PFF

• Excellent passivation quality of the poly-Si layer

high Voc

• Good transport through the poly-Si / c-Si junction

low Rs

Area

[cm²]

Voc

[mV]

Voc [mV]

(full area

illumination)

PFF

[%]

FF

[%]

Rs,FF

[Ωcm²]

Jsc

[mA/cm²]

η

[%]

4.25 705 714 73.1 71.2 0.6 28.8 14.5

Documents

Polycrystalline silicon as carrier selective contact for silicon solar cells · 2018. 8. 12. · Polycrystalline silicon as carrier selective contact for silicon solar cells SPREE